Compositions and methods for scale inhibition

ABSTRACT

A method for inhibiting the formation, deposition and adherence of scale to metallic and other surfaces in the equipment, vessels and/or piping of facilities for the handling of oil and gas produced fluid is disclosed. An effective scale inhibiting amount of alginate is added to a produced fluid containing a scale-forming divalent cation. The alginate effectively cross-links with a scale-forming divalent cation, e.g., calcium, forming an alginate gel for subsequent separation and removal from the produced fluid.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims benefit under 35 USC 119 of U.S. ProvisionalPatent Application No. 62/111,312 with a filing date of Feb. 3, 2015,which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to compositions and methods to inhibitscale formation, deposition, and adherence in equipment and facilitiesin oil & gas operations.

BACKGROUND OF THE INVENTION

When an oil or gas well produces water (generally with a large contentof dissolved salts), there is a possibility for scale to form. This mayalso occur in facilities where water injection is used as an improvedrecovery system, or when using gas with high CO₂ content and othercomponents. Buildup of mineral deposits or incrustations may occur inpipes and equipment both on the surface and in the bottom of the well,or even inside the porous medium in the formation of the oil deposititself. Buildup of scale deposits can result in significant reduction inoil production or even full blockages in pipes, and exacerbate corrosionon the surfaces of equipment used to handle and process such producedfluid.

As an example, oil and gas pipelines, including subsea pipelines,typically carry production fluids from the production wells (includingsubsea wells). The presence of calcium in the produced water fromhydrocarbon extraction can result in the formation of inorganic andorganic salts (known as scales) that may impede production ofhydrocarbons or limit the ability to manage water in surface facilitiesand subsurface injection facilities. Ongoing deposition of scale inpipelines and equipment can result in production and injectionimpairment, flow restrictions, process upsets, and operational issues(e.g. malfunction of valves).

One common method for scale control is the injection of scale inhibitorchemicals. Scale inhibitors prevent the formation of large deposits ofmineral scale that would otherwise form rapidly when the brine isoversaturated with respect to a specific scale type. The use of scaleinhibitor chemicals for calcium scale control is most common in brineswith low to moderate pH of ˜4-7.

The application of scale inhibitor treatment is varied according to thelocation. Despite the widespread use of scale inhibitors, there are anumber of operating conditions where the use of conventional scaleinhibitors may not be viable and alternative strategies are required.

There is a need for improved compositions and methods to inhibit scaleformation, deposition, and adherence thereof.

SUMMARY

A method for preventing scale formation is disclosed.

In one aspect, the invention relates to a method of inhibiting theformation of scale on equipment in contact with a produced fluidcontaining at least a scale-forming divalent cation. The methodcomprises: adding an alginate in an amount effective for the alginate tocross-link with the divalent cation in the produced fluid; separatingthe cross-linked alginate gel in the produced fluid; removing thecross-linked alginate gel from the produced fluid; and redissolving thecross-linked gel thus returning it to a reusable form.

In another aspect, the invention relates to a method of inhibitingcalcium scale formation on equipment in contact with a produced fluid.The method comprises: determining the calcium scale forming profile ofthe produced fluid, including concentration and characteristics ofcalcium ions in the produced fluid; selecting an alginate having apre-select M/G ratio to cross-link with the calcium ions in the producedfluid; adding the alginate having a pre-select M/G ratio in an amounteffective for the alginate to cross-link with the calcium ions in theproduced fluid to form cross-linked calcium alginate gel; separating thecross-linked calcium alginate gel in the produced fluid; removing thecross-linked calcium alginate gel from the produced fluid; andredissolving the cross-linked calcium alginate gel thus returning it toa reusable form.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating scale control in a base design case,for a pH stabilized pipeline that does not carry formation water wherepH stabilization is used to control the corrosion rate of the pipeline.

FIG. 2 is a diagram illustrating scale deposition in a pH stabilizedpipeline carrying formation water.

FIG. 3 is a diagram illustrating an embodiment for scale control withalginate in a pH stabilized pipeline carrying formation water.

FIG. 4 is a diagram illustrating using alginate to mitigate scaleformation in brine to be used for hydraulic fracturing applications.

FIG. 5 is a diagram depicting the results of laboratory experimentsdemonstrating the effectiveness of alginate in reducing calcium ionconcentration in one example.

FIG. 6 is a diagram illustrating an embodiment of a process for thecreation and resolution (e.g., redissolving) of the cross-linked gel,returning it to a reusable form.

DETAILED DESCRIPTION

The following terms will be used throughout the specification and willhave the following meanings unless otherwise indicated.

“Comprise”, “have”, “include” and “contain” (and their variants) areopen-ended linking verbs and allow the addition of other elements whenused in a claim.

“Effective amount,” refers to an amount sufficient to effect ameasurable difference over not including the amount.

“Scale,” “calcium scale” and “calcium salt scale” as used in thespecification and claims herein shall include without limitation allscale consisting of insoluble salts formed from divalent cations in theprocesses described herein, e.g., calcium, magnesium, barium, strontium,etc.

“Scaling” relates to the formation of “scale” as defined above.

“Inhibitor” or “inhibition” as used herein means that scale formationquantity is reduced or the character of the scale formation is changed,e.g., the adherent nature of the scale is reduced, or the scale becomessemi-solid or as a gel.

“Produced fluids” may be used interchangeably with produced waters,referring to mixtures of hydrocarbons and water that is typicallyextracted with the hydrocarbons from a formation. As used herein,produced fluids also include fracture fluids, which can be recycledproduced water or water from co-located wells for use in creatingfractures in shale formation (sometimes can be referred to as “brine”).Produced fluids may also contain glycols that have been injected intothe well fluids to prevent hydrate formation. Produced fluids mayinclude formation water (e.g., connate water), produced water,hydrocarbon liquids, gas, or any combination thereof. Also, sometimesthe terminology “produced fluids” and “produced fluid” is usedinterchangeably herein.

The presence of divalent cations such as calcium and magnesium inproduced fluids can result in the formation of inorganic and organicsalts (known as scales) that may impede production/extraction ofhydrocarbons, or limit the ability to manage water in surface facilitiesand subsurface injection facilities. Disclosed is a method and acomposition to inhibit scale formation, deposition, and adherencethereof in equipment, facilities, and operations for the exploration,production, and processing of hydrocarbons.

Types of Scale Formation:

The scale formation to be inhibited can be inorganic, organic, and/ormixtures thereof. Inorganic scales can deposit throughout the entireproduction and processing system used for oil & gas production. The mostcommon type of inorganic scale formed in oil and gas operations iscalcium carbonate. The driving forces for calcium carbonate depositionare primarily increases in pH and increases in temperature, as in theprior art, the use of scale inhibitor chemicals for calcium scalecontrol is most common in low to moderate pH environments.

Organic scales such as calcium carboxylate or naphthenate salts are lesscommon than inorganic scales, but can cause severe processing andmaintenance issues in surface facilities. The formation of these saltscan occur during the production of hydrocarbons that contain naphthenictype acids in conjunction with produced waters that contain calcium.During the production and processing of these fluids, the reduction inpressure causes the pH of the brine to increase as a result of thedissolution of acidic gasses, thus allowing the naphthenic acid speciesto form salts with the calcium ions.

Scale Inhibition Composition:

The composition for use in inhibiting the formation of scales is analginate composition with an affinity for divalent cations (e.g.,calcium) and the ion-complexing properties of the alginates, thusobviating the formation of the solid inorganic and or organic scales.Alginates have been primarily used in the biomedical industry, e.g.,creating a moist healing environment for the management of chronicwounds; or used as a stabilizer, thickener, and emulsifier in the foodindustry; or for use in biomedical applications as a diet-aidsupplement. The unique complexing and gel forming properties ofalginates can be conveniently used to inhibit the formation andadherence of solid scales for oil & gas applications.

Alginates provide the main structural component of brown algae(seaweeds). Alginates are linear copolymers of (1-4) linkedβ-d-mannuronic acid (M) and α-1-guluronic acid (G). The distribution ofM and G in alginate chains gives rise to three different block types,namely blocks of poly-M, blocks of poly-G and alternating MG blocks. Thecharacteristics of alginate can be dictated by the content of the Gblocks, and also their length. Alginate's ability to form gels arisesfrom divalent cations or other multivalent ions fitting into the G-blockstructure, binding the alginate monomers together to form a continuousnetwork.

The chemical composition of alginate is variable according to theseaweed species, within different parts of the same plant (stem orleaf), seasonal changes and the conditions of the sea. Alginatesmolecular arrangement and composition are determined primarily by thesource from which they are obtained. For example, an alginate derivedfrom Macrocystis pyrifera has an M/G ratio of ˜1.56:1; and an alginatederived from Laminaria Hyperborea has an M/G ratio equal to 0.45. Thecommon, commercially available alginates have a G content ofapproximately 40%. A high content of G-blocks gives an alginate of highaffinity and selectivity for polyvalent cations.

The affinity for cations and the gel forming properties of thealginates, i.e., producing a cross-linked structure with cations, aremostly related to the content of G residues, as when two G residues areadjacent in the polymer they form a binding site for polyvalent cations.The gel-forming involves the binding or chelating of the scale-formingions such as magnesium, barium, calcium, etc., inside the structure oftwo guluronic acid (G blocks) of the alginate structure.

A gel, in classical colloid terminology, is a system which owes itscharacteristic properties to a cross-linked network of polymer chainswhich form at the gel point. The alginate gels formed consist of highlyhydrated alginate polymers. By proper selection of the alginate gellingagent, gel structure and rigidity can be controlled. Soft gels tend toflow and assume the shape of their container. Alginates from Laminariahyperborea seaweed with a large percentage of the G-blocks, form rigid,brittle gels which tend to undergo syneresis, or loss of bound water. Incontrast, alginate from Macrocystis pyrifera or Ascophyllum nodosumforms elastic gels which can be deformed with a reduced tendency towardsyneresis.

In one embodiment, an alginate composition has a pre-selected M/G ratiosuitable for the environment where the alginate is injected and optimumfor the resultant physical properties of the 1 cross-linked structure(i.e., the resulting gel) such as gel strength, shear thinning andviscosity. Factors affecting the gel properties include the composition,pH and temperature of the produced fluids as well as the concentrationand type of scale forming materials (e.g., calcium or magnesium ions,inorganic or organic scale forming materials).

In one embodiment, the alginate has a ratio of mannuronic acid toguluronic acid (M/G) of any of less than 1; less than 0.7; and 0.5. Inanother embodiment for use with produced fluids having a low pH, thealginate has a larger amount of guluronic acid compared to mannuronicacid, e.g., where guluronic acid is above 60% and the amount ofmannuronic acid is below 40% of the total content of alginate. In yetanother embodiment, the ratio of beta-D mannuronic acid to alpha-Lguluronic acid in the high viscosity alginates is equal to or above 1.In one embodiment, wherein it is desirable for the cross-linked gels (orflocs) to be sufficiently strong to be separated from the producedfluids by mechanical methods such as cyclone separation, the alginatehas a G-content above any of 30%; 40%; and 50%. In one embodimentwherein most of the scale formation is expected from barium or strontiumions, the alginate employed has an M/G ratio ranging from 0.01 to 0.8.

In yet another embodiment, the alginate has a G-content of more than orequal to 50% (e.g., at least 50%). For example, a G-content of more thanor equal to 50% (e.g., at least 50%) may be used where scale formationis expected to be from calcium. In yet another embodiment, the alginatehas a G-content of more than or equal to 60% (e.g., at least 60%). Inyet another embodiment, the alginate has a G-content of more than orequal to 70% (e.g., at least 70%). In yet another embodiment, thealginate has a G-content of 50% to 80%. In yet another embodiment, thealginate has a G-content of 50% to 70%. In yet another embodiment, thealginate has a G-content of 50% to 60%. In yet another embodiment, thealginate has a G-content of 50% to 55%. In yet another embodiment, thealginate has a G-content of 55% to 60%. In yet another embodiment, thealginate has a G-content of 60% to 80%. In yet another embodiment, thealginate has a G-content of 60% to 70%. In yet another embodiment, thealginate has a G-content of 60% to 65%. In yet another embodiment, thealginate has a G-content of 65% to 75%. In yet another embodiment, thealginate has a G-content of 55% to 70%.

In one embodiment, the alginate has a M/G ratio to cross-link with thedivalent cation, and wherein the G-content of the alginate is at least(or is a minimum of) 50% which is equivalent to the M/G ratio of thealginate being a maximum of 1.

A G-content in alginate of 50% may be considered to be a 1:1 ratio. Inone embodiment, the alginate has a maximum M/G ratio of 1:1.

In one embodiment for the prevention of calcium forming scale, thealginate has a weight ratio of M/G block ranging from 0.33:1 to 1:1. Inanother embodiment, the alginate has a weight ratio of M/G block from0.45:1 to 1:1. In another embodiment, the alginate has a weight ratio ofM/G block from 0.33:1 to 0.75:1. In another embodiment, the alginate hasa weight ratio of M/G block from 0.45:1 to 0.75:1. In anotherembodiment, the alginate has a weight ratio of M/G block from 0.5:1 to0.75:1. In another embodiment, the alginate has a weight ratio of M/Gblock from 0.6:1 to 0.75:1. In another embodiment, the alginate has aweight ratio of M/G block from 0.70:1 to 0.75:1.

In another embodiment, the alginate is added to the produced fluid as asolution of 0.3 to 5% alginate (e.g., as sodium alginate), and at aratio of sodium ions to calcium ions at a ratio ranging from 10:1 toabout 50:1. In yet another embodiment, the alginate is added as asolution of less than or equal to 3% alginate (e.g., as sodiumalginate). In another embodiment, the alginate is added as a solution ofless than or equal to 2% alginate (e.g., as sodium alginate). In anotherembodiment, the alginate is added as a solution of less than or equal to1% alginate (e.g., as sodium alginate). The upper limit can bedetermined by the physical characteristics of the alginate which canstart to become increasingly viscous and insoluble.

In one embodiment, the alginate composition is a mixture of differentalginates with different M/G ratios and/or different viscosities toprovide the desired effects, e.g., the composition will be readilysoluble in water, such that the composition can be easily used toprepare an aqueous preparation without substantive mixing. The alginatecomposition can be in the form of a solution or a water solublemonovalent salt of alginate (e.g., Na salt, K salt, or NH₄ salt),slurried and/or dissolved in a carrier fluid e.g., water, mono ethyleneglycol (MEG), etc., in a sufficient amount for the alginate toeffectively cross-link with the scale-forming cations in the producedfluid. A sufficient amount means an amount for the alginate toeffectively cross-link with at greater than 75% cross-linking in oneembodiment; at least 90% cross-linking in a second embodiment; at least95% cross-linking in a third embodiment; and at least 99% cross-linkingin a fourth embodiment.

In one embodiment, alginate can be used to inhibit scale when thequantity of divalent cations in the produced fluid is between about zerodivalent cations (e.g., less than 10 ppm) and saturated produced fluidof divalent cations (e.g., calcium ions). For example, an upper limitmay be determined by the saturation point for the cations (e.g., calciumions) which is dependent on fluid composition and conditions.

As most if not all of the scale forming materials in the producedfluids, e.g., the calcium ions, the magnesium ions, the barium ions,etc., react with the alginate to form gels instead of hard-scale onpipes and equipment, the occurrence of scale on equipment issignificantly reduced if not prevented. Scale formation is reduced atleast 75% compared to an occurrence without any alginatetreatment/addition in one embodiment; at least 90% reduction in a secondembodiment; at least 95% reduction in a third embodiment; and at least99% reduction in a fourth embodiment.

Optional Additives: In one embodiment, an optional amount of additives,e.g., a preservative such as benzoic acid, potassium sorbate, sodiumbenzoate is added to the alginate composition to prevent the microbialgrowth. In yet another embodiment, a sufficient amount of a base isadded to the alginate composition/carrier fluid (e.g., MEG) to adjustthe pH for a desired cross-linked structure with suitable rheologicalproperties for transport through the pipelines.

Methods for Inhibiting Scale Formation: In one embodiment, the amountand type of alginates to be injected for scale inhibition can bedetermined from a reference database. The reference database is used tocharacterize the scale inhibition characteristics of a selectedalginate, as well the optimization and blending of alginate compositionsfor optimal scale inhibiting results. The reference database can containcorrelations of any of the alginate properties, e.g., ratio of the M&Gblocks of alginates, viscosity, and solubility with; a) properties ofproduced fluids including but not limited to the uptake of divalentcations, gel strength, viscosity, the amount of monovalent salts in theproduced fluid; and b) properties of cross-linked gel products.

In one embodiment, the divalent cation scale forming profile in theproduced fluid is first determined, along with the characteristics ofthe produced fluid, e.g., pH, operating temperature, components, ironlevel, etc. In the next step based on the correlations, an alginate witha particular M/G ratio is selected to enhance the functionality of thealginate to minimize the crystal growth of the scale by maximizing theuptake of scale-forming divalent cations, e.g., Ca++, from the producedwater into a calcium-alginate cross-linked structure, i.e., gels, withsuitable rheological properties for transport through pipelineinfrastructure or for subsequent prevention using methods known in theart.

In one embodiment for the prevention/control of scale formation in apipeline, the alginate can be introduced into the pipeline by itself, orwith a carrier fluid such as MEG, into the production well at the wellhead, into a manifold, into a location downhole in the wellbore, at anintermediate location of the pipeline between the production well and aprocessing facility (e.g., brine treatment facility, glycol treatmentfacility or glycol processing plant, etc.), at intervals along thepipeline, or any combination of the above.

The cross-linked gels/agglomerates/complexes may be removed downstreamand the alginate recovered for reuse by any combination of mechanicaland chemical means known in the art, e.g., by settling, sieving, using acentrifuge or cyclone, pH adjustment and elution.

Scale Inhibiting Applications:

The inventive method to use alginates for scale inhibition and controlcan be particularly advantageous for conditions where the use of commonscale inhibitors may not be viable. These scenarios include but are notlimited to: a) operations with high pH, high saturation ratio or hightemperatures or in areas where the brine is re-used or re-injected; b)systems where corrosion is managed by a ‘pH stabilization’ method; c)operations and processes for the preparation, storage and use offracture fluids; d) glycol distillation system for the pre-treatment ofproduced fluids; e) operations wherein the scale comprises calciumnaphthenate/carboxylate salts, solids and emulsions; and processesinvolving high iron, oxygenated systems; f) sour water stripping system;g) systems with high contents of both Ca and SO₄ to result in highscaling risk of CaSO₄ based minerals; h) operations where the producedfluid includes sulfur; i) operations where the produced fluid includessulfates; j) other operation; k) any combination thereof; l) etc.

Scale Inhibition in Gas Production Pipelines Employing pH Stabilization:

pH stabilization can be used as a method to control CO₂ corrosion in gasproduction pipelines. In this technique, a high pH base (e.g., NaOH,MDEA, KOH, etc.) is used to promote the formation of iron carbonatescale as a passivating film to protect the internal pipe wall fromon-going corrosion. A prerequisite for pH stabilization method is thatthe pipeline does not carry formation water, since the presence ofcalcium ions from formation water may lead to a calcium carbonateformation in the elevated pH environment of the system. In the prior artwith the use of scale inhibitors for pH stabilized systems, the kineticsof iron carbonate deposition can be affected with growth and resultantdensity of the iron carbonate passivating layer, which could compromisethe ability of the pH stabilization method to control corrosion in thepipeline. The use of alginates as a scale inhibition/control methodobviates the scale formation while allowing the formation of the ironcarbonate passivating layer.

Preparation of Fracture Fluids:

Alginates can also be used for controlling/inhibiting scale formation inthe preparation, storage and use of fracture fluids. In a typicalhydraulic fracturing operation, there is a need to use recycled producedwater or alternatively, co-located water source wells to supply brinefor the preparation of fracture fluids. This often leads to the use ofwaters that contain divalent ions (e.g., calcium, magnesium, etc.) forpreparation of the final fracture fluid. If levels of divalent cationsin the brines are high enough, this can result in scale depositionoccurring in equipment during the preparation of the fracturing fluid.Scale deposition would lead to equipment downtime and cleaning, and maynecessitate the use of acids or other scale solvents to remove thedeposits. If the scale were to deposit downhole during the use of thefracture fluid, this may reduce post-frac production from the well. Inthe prior art, water to be used as “brine” may be pre-treated or“softened” to remove divalent cations such as calcium prior to its use,with techniques such as lime softening and/or ion exchange to reduce thecalcium level sufficiently to prevent inorganic scale precipitation. Theuse of alginates as a scale inhibition/control method obviates the waterpre-treatment step.

Glycol Distillation Systems:

In the prior art method for preventing scaling of glycol distillationsystems, prior to distillation of rich glycol (e.g., a mixture ofproduced water and glycol typically containing 40-70% MEG), largevolumes of alkali chemicals (e.g., NaOH, KOH, K₂CO₃, etc.) are added torich glycol in a heated pre-treatment system to forcibly precipitate andremove divalent salts. In one embodiment, alginate is used to removecalcium and other divalent ions from the rich glycol, thus obviating theneed for the handling of large volumes of alkali chemicals and therequirement for heating the rich glycol prior to distillation.

Systems with Organic Scales & Emulsions.

During the production and processing of hydrocarbons that containnaphthenic type acids in conjunction with produced waters that containdivalent cations such as Ca²⁺ or Mg²⁺, the reduction in pressure cancause an increase in the pH of the brine, allowing the naphthenic acidspecies to form salts with the divalent cations. The presence ofnaphthenate salts result in emulsions and solid deposition. Commerciallyavailable scale inhibitors are not effective on these types of organicsalts; hence the scale formation is typically controlled by processoptimization or by the use of large volumes of acid to suppress the pH.The use of alginates to control/inhibit the organic scale formationobviates the need for process optimization in the prior art, which mayinclude controlling the rate of depressurization of the fluids,injection of demulsifiers, and/or injection of organic or mineral acidsinto the produced fluid stream to maintain the pH to prevent calciumnaphthenate formation.

High Iron/Oxygenated Systems:

Many common scale inhibitor products are ineffective when Fe²⁺ levelsexceed 50-100 ppm in produced fluids. In addition, Fe³⁺ will also rendercommon scale inhibitors ineffective if present in the produced fluids atlevels >1 ppm. The use of alginates facilitates the control/inhibitionof scale formation in high iron, oxygenated systems, obviating the needfor remedial treatment such as pipeline removal or chemical cleaning toremove scale formation.

Sour Water Stripping Systems:

Sour water stripping is widely used to remove sulfides from producedwater so that it can be safely and reliably disposed of, re-injected, orre-used after proper treatment. The sour water is sent through astripping tower where a gas (usually steam) stream is applied to forceH₂S and in the meanwhile, CO₂, out of the water phase. By removing theseacidic components from the produced water, the process usually resultsin increased pH, and consequently leads to a higher scaling risk ofCaCO₃. In some scenarios seen in the field, the Ca concentration in theproduced water is so high that commercial inhibitors, such as NTMP,would be ineffective. Ca removal through pre-precipitation (softening)or pH adjustment of the produced water is usually recommended to controlsuch scaling risk in the prior art. The use of alginates reduce theavailability of Ca to form CaCO3 precipitation, and could thus obviatethe needs for these pretreatment processes.

High Ca/SO₄ systems: Although CaSO₄ scaling is not as common as BaSO₄and CaCO₃ due to its relatively high solubility, it has been seen in thefield that systems with high concentrations of both Ca and SO₄ can havesignificant scaling risk of CaSO₄ based minerals (e.g., gypsum,hemihydrite, anhydrite, etc.) that are difficult to inhibit due to thelarge amounts of potential precipitation. If pH is lowered in the samesystem to control the scaling of carbonates/sulfides, the availabilityof Ca is further increased to induce more CaSO₄ scale, which is notsensitive to change in pH. Ca reduction might be the only viable optionin these systems. Although processes such as softening precipitation ormembrane filtration can be utilized to pre-treat the water, alginatescould be dosed in the same water to chelate Ca and obviate thesepretreatment needs.

Figures Illustrating Embodiments

Reference will be made to the figures to further illustrate embodimentsof the invention. Of note, in FIGS. 1-4, the produced fluids maycomprise gas and hydrocarbon liquids, produced water (e.g., brine), andformation water (e.g., connate water). The different terminology is usedfor simplicity, but the scope of the claims should not be limited by theuse of the different terminology in the embodiments of this disclosure.

FIG. 1 is a diagram illustrating an embodiment 100 of a base design caseof a pH stabilized pipeline for the transport of produced fluids withoutany formation water. In this operation, a pH stabilizing agent (e.g.,MDEA, KOH, NaHCO₃, etc.) is added to the produced fluids 105 (e.g., gas,hydrocarbon liquids) at 120 to raise the pH of <6.5 in section 110 to apH of >6.5 in section 115, with the formation of iron carbonatepassivating scale to provide corrosion protection for the pipeline.

In FIG. 2 for the same operation, an embodiment 200 includes producedfluids 205 that comprise formation water (e.g., connate water)containing calcium and other ions at 220. There is moderate to low levelof CaCO₃ scale forming/deposition due to unmitigated calcium in theproduced fluids in the section 210 with pH<6.5. As illustrated, afterthe injection of pH stabilizing agent at 225, a very high quantity ofCaCO₃ deposit/scale is observed due to unmitigated calcium in theproduced fluids under the high pH of >6.5 in section 215.

FIG. 3 illustrates an embodiment 300 of the invention for the inhibitionof scale formation/adherence in the operation of FIG. 2. As in FIG. 2,the embodiment 300 includes produced fluids 305 that comprise formationwater containing calcium and other ions at 320, a section 310 withpH<6.5, and a section 315 of pH >6.5 after the injection of pHstabilizing agent at 330. With the injection of alginate at 325, thealginate cross-links with calcium forming a gel 335/viscous phasecontaining calcium ions (which would otherwise have formed CaCO₃deposit/scale).

FIG. 4 illustrates an embodiment for the use of alginate in brinetreatment applications, e.g., the treatment of brine (“produced water”)for use in fracturing applications. In one embodiment 400, brinecontaining calcium (and other ions) is discharged into a pond/tank at405. At 410, alginate is injected into the inlet of a discharge pumpalong with the brine, or it can be discharged into the pond/tank wherethe brine is stored. The alginate cross-links with calcium in the brineat 415, forming a gel layer that settles at the bottom of the pond/tankat 420. The settling of the gel (e.g., cross-linked calcium alginategel) in the brine is an example of separating the cross-linked alginategel in the produced fluid. For example, the various components of thebrine can separate by phases within the pond/tank, with the lightestcomponents towards the top, and the heavier gel settles towards the baseof the pond/tank. Thus, these components are splitting by phase. In someembodiments, separating the cross-linked alginate gel in the producedfluid can be phase separating the cross-linked alginate gel in theproduced fluid.

At 425, calcium-free brine is removed/pumped from the pond/tank for usein preparing frac water. The frac water can be prepared by addingproppants and other components. Removing the calcium-free brine from thepond/tank and leaving behind the gel that has settled is one example ofremoving the gel from the produced fluid. In another embodiment, the gelis not left behind and can also be removed from the pond/tank. In yetanother embodiment, the gel is removed first and the brine is left inthe pond/tank. Nonetheless, the gel is removed from the calcium-freebrine.

Additionally, ions can be removed from the gel that has settled, and thegel can be redissolved, as explained further in FIG. 6. For example, thegel that has settled in FIG. 4 can be the gel at 615 of FIG. 6.

In an alternative embodiment, FIG. 4 can be modified to inhibit scaleformation using alginate and also remove or reduce sulfur from theproduced fluids. In this alternative embodiment, the produced water at405 of FIG. 4 can include sulfur in any quantity (often referred to assour water). The sulfur level can be measured via inductively coupledplasma (ICP). In this alternative embodiment, the pond/tank at 410 canbe practically any pond or tank, and it does not need to be specific forfracturing. In this alternative embodiment, at 425, the calcium-freebrine is pumped from the pond/tank and sent to a sulfur stripper forremoval of the sulfur. The sulfur stripper can be practically any sulfurstripper used or known by those of ordinary skill in the art for theremoval of sulfur. After the sulfur has been removed, for example, thecalcium-free brine can be pumped for general use or disposal. In thisalternative embodiment, the produced fluid requires treatment in asulphur stripper, and the process used to remove sulphur can cause thescale to form. The scale formation can be inhibited by the addition ofthe alginate.

In an alternative embodiment, FIG. 4 can be modified to inhibit scaleformation using alginate when the produced fluids include sulfate. Inthis alternative embodiment, the produced water at 405 of FIG. 4 caninclude sulfate, such as a high sulfate level. In one example, thesulfate level is more than or equal to 1500 mg/l. In another example,the sulfate level is more than or equal to 2000 mg/l. In anotherexample, the sulfate level is in a range of 1500 mg/l to 2000 mg/l. Thesulfate level can be measured via EPA Method 9038. Alternatively, thesulfate level can be measured using a sulfate test kit, such as a kitavailable from Hach Company, PO Box 389, Loveland, Colo. 80539. In thisalternative embodiment, the sulfate level is not necessarily altered andthe focus is removing Ca so that in the case of high SO4 (e.g.,SO4>2,000 mg/l or so), the alginate can prevent unwanted precipitationand deposition of CaSO4 scale. In this alternative embodiment, thepond/tank at 410 can be practically any pond or tank, and it does notneed to be specific for fracturing. In this alternative embodiment, at425, the calcium-free brine is pumped from the pond/tank for general useor disposal. Thus, this alternative embodiment reduces the risk ofcalcium sulfate.

Various other alternatives or modifications are also possible. Forexample, the separating and removal steps may be accomplished in otherways. In one example, separation of the gel from the fluids could be bysettling in a pond/tank and then removal of the gel from the pond/tank,with the fluids remaining in the pond/tank or overflowing from thepond/tank (e.g., fluids overflowing into another container). In anotherexample, a form of filtration or cyclonic separation can be used wherethe fluids and the gel undertake simultaneous separation and removal. Inanother example, removing the cross-linked gel from the produced fluidis more about removing the calcium/divalent cations from the system(e.g., as part of redissolving the gel thus the removing step and theredissolving step may be combined into one step). Depending on theembodiment, the separating step, removing step, and redissolving stepscan be performed as three separate steps. Depending on the embodiment,some or all of the separating step, removing step, and redissolving stepcan be combined. Depending on the embodiment, at least one of theseparating step, removing step, or redissolving step can be omitted.Also, as indicated above, the separating step can be a phase separatingstep in some embodiments. Those of ordinary skill in the art willappreciate that the inventive concepts are not limited to the examplesprovided herein.

FIG. 5 is a diagram depicting the results of laboratory experimentsdemonstrating the effectiveness of alginate in reducing calcium ionconcentration in one example 500. In example 500, the addition of 15 mlof the 1% alginate solution can lead to a drop in calcium ionconcentration of about 70% (from 100% to 30%) from the original 300 ppmsolution, after allowing/normalising for the dilution effect of addingthe alginate.

FIG. 6 is a diagram illustrating an embodiment of a process for thecreation and resolution (e.g., redissolving or redissolution) of thecross-linked gel, returning it to a reusable form. A process 600 startswith creating an alginate solution at 605. As illustrated at 605, asodium alginate solution can be created with a pH that is basic. At 610,the process 600 includes adding the sodium alginate solution to producedfluids, such as produced water, having divalent cations, such as calciumions. At 615, the process 600 includes cross-linking between thealginate and the calcium ions to form a gel (e.g., a cross-linkedcalcium alginate gel) with a pH that is basic. The cross-linked calciumalginate gel is a solid or has a substantially solid consistency. Thecalcium ions would have formed deposit/scale without the alginatesolution.

At 620, the process 600 includes adding an acid to the formed gel. Forexample, 0.1M hydrochloric acid can be added to the cross-linked calciumalginate gel to convert the calcium alginate into hydrogen alginate(alginic acid), resulting in a hydrogen alginate gel with a pH that isacidic. As illustrated at 627, the majority of calcium ions in thehydrogen alginate gel can be removed in process 600 as a result of theconversion of the calcium alginate into hydrogen alginate by theaddition of acid. Of note, other acids or acid quantities can be used inother embodiments.

At 630, the process 600 includes adding washing fluid (e.g., washingwater) to the hydrogen alginate gel to neutralize the pH. The washingwater can be practically any type of water, fresh water, or fluid thatcan help neutralize the pH of the hydrogen alginate gel. A hydrogenalginate gel with a pH that is neutral is illustrated at 635.Furthermore, at 640, the addition of the washing water removes or washesaway the residual calcium ions from the hydrogen alginate gel.

At 645, the process 600 includes adding a salt to the hydrogen alginategel. For example, 0.1M sodium hydroxide can be added to the hydrogenalginate gel so that the alginate is redissolved in the form of thesoluble salt, sodium alginate, that can be used to create the sodiumalginate solution at 605.

As illustrated in FIG. 6, calcium ions are removed at two points in theprocess 600: (A) calcium ions are removed where the acid is added whichconverts the calcium alginate to hydrogen alginate (alginic acid) (620to 627) and (B) calcium ion are removed again where the washing waterremoves or washes away the remaining the calcium ions (630 to 640).Furthermore, as mentioned above, by adding the sodium hydroxide, thealginate is redissolved in the form of the soluble salt, sodiumalginate, that the process 600 starts with at 605. In some embodiments,redissolving the cross-linked gel thus returning it to a reusable formcomprises one or more of: adding an acid to the cross-linked gel tochange pH of the gel from basic to acidic; adding a washing fluid to thecross-linked gel to change the pH of the gel from acidic to neutral;and/or adding a salt to the cross-linked gel with the neutral pH to forman alginate. Therefore, at least one of these three steps can be part ofredissolving. In some embodiments, at least one divalent ion is removedfrom the cross-linked gel in response to the addition of the acid, inresponse to the addition of the washing fluid, or both. The gel may beredissolved more effectively by removing calcium and/or divalentcations.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims are to be understood as being modified in all instances by theterm “about”. Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that can vary depending upon thedesired properties sought to be obtained by the present invention. It isnoted that, as used in this specification and the appended claims, thesingular forms “a”, “an”, and “the”, include plural references unlessexpressly and unequivocally limited to one referent.

As used herein, the term “include” and its grammatical variants areintended to be non-limiting, such that recitation of items in a list isnot to the exclusion of other like items that can be substituted oradded to the listed items. The terms “comprises” and/or “comprising,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. Unless otherwise defined, all terms, including technical andscientific terms used in the description, have the same meaning ascommonly understood by one of ordinary skill in the art to which thisinvention belongs.

As used herein, terminology such as A, B, C, or any combination thereof(or the like such as A, B, C, or any mixtures thereof) relate to variousoptions. In one embodiment, the terminology A, B, C, or any combinationthereof means A only. In one embodiment, the terminology A, B, C, or anycombination thereof means B only. In one embodiment, the terminology A,B, C, or any combination thereof means C only. In one embodiment, theterminology A, B, C, or any combination thereof means A and B only. Inone embodiment, the terminology A, B, C, or any combination thereofmeans B and C only. In one embodiment, the terminology A, B, C, or anycombination thereof means A and C only. In one embodiment, theterminology A, B, C, or any combination thereof means A, B, and C.Moreover, an embodiment can have a single A or a plurality of A. Anembodiment can have a single B or a plurality of B. An embodiment canhave a single C or a plurality of C.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope is defined bythe claims, and can include other examples that occur to those skilledin the art. Such other examples are intended to be within the scope ofthe claims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims. All citations referred herein are expressly incorporatedherein by reference. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

1. A method of inhibiting formation of scale on equipment in contactwith a produced fluid containing at least a scale-forming divalentcation, the method comprising: adding an alginate in an amount effectivefor the alginate to cross-link with the divalent cation in the producedfluid to form cross-linked alginate gel; separating the cross-linkedalginate gel in the produced fluid; removing the cross-linked alginategel from the produced fluid; and redissolving the cross-linked gel thusreturning it to a reusable form.
 2. The method of claim 1, wherein theproduced fluid contains calcium carbonate, calcium carboxylate, calciumnaphthenate, or any mixture thereof.
 3. The method of claim 1, whereinthe scale forming divalent cation is calcium and wherein alginate isadded in an amount sufficient to convert at least 75% of calcium cationsinto a cross-linked calcium alginate gel.
 4. The method of claim 1, forinhibiting the formation of scale in a pipeline carrying produced fluid.5. The method of claim 1, for inhibiting the formation of scale in abrine or glycol treatment facility, wherein the produced fluid isrecycled produced water for use in hydraulic fracturing operations. 6.The method of claim 1, wherein the produced fluid is recycled producedwater and wherein the cross-linked alginate gel is removed from therecycled produced water by floatation, sieving, a centrifuge, a cyclone,a gravity settling device, or any combination thereof.
 7. The method ofclaim 1, wherein the produced fluid requires treatment in a sulphurstripper.
 8. The method of claim 7, further comprising removing thesulfur from the produced fluid after the cross-linked alginate gel hasbeen removed from the produced fluid.
 9. The method of claim 1, whereinthe produced fluid comprises sulfate.
 10. The method of claim 9, whereinthe sulfate in the produced fluid is more than or equal to 1500 mg/l.11. (canceled)
 12. The method of claim 1, wherein the alginate is addedto the produced fluid before a pH stabilization agent is added to theproduced fluid.
 13. The method of claim 1, wherein the alginate has aM/G ratio) to cross-link with the divalent cation, and wherein theG-content of the alginate is at least 50% which is equivalent to the M/Gratio of the alginate being a maximum of
 1. 14. The method of claim 1,wherein the alginate is added to the produced fluid in a solution, andwherein the alginate is 0.3% to 5% of the solution.
 15. The method ofclaim 1, wherein redissolving the cross-linked gel thus returning it toa reusable form comprises at least one of: adding an acid to thecross-linked gel to change pH of the gel from basic to acidic; adding awashing fluid to the cross-linked gel to change the pH of the gel fromacidic to neutral; or adding a salt to the cross-linked gel with theneutral pH to form an alginate.
 16. The method of claim 15, wherein atleast one divalent ion is removed from the cross-linked gel in responseto the addition of the acid, in response to the addition of the washingfluid, or both.
 17. A method of inhibiting calcium scale formation onequipment in contact with a produced fluid, the method comprising:determining the calcium scale forming profile of the produced fluid,including concentration and characteristics of calcium ions in theproduced fluid; selecting an alginate having a pre-select M/G ratio) tocross-link with the calcium ions in the produced fluid; adding thealginate having a pre-select M/G ratio in an amount effective for thealginate to cross-link with the calcium ions in the produced fluid toform cross-linked calcium alginate gel; separating the cross-linkedcalcium alginate gel in the produced fluid; removing the cross-linkedcalcium alginate gel from the produced fluid; and redissolving thecross-linked calcium alginate gel thus returning it to a reusable form.18. The method of claim 17, wherein the G-content of the alginate is atleast 50% which is equivalent to the M/G ratio of the alginate being amaximum of
 1. 19. The method of claim 18, wherein the G-content of thealginate is 50% to 80%.
 20. The method of claim 17, wherein the M/Gratio is in a range of 0.33:1 to 1:1.
 21. (canceled)
 22. The method ofclaim 17, wherein redissolving the cross-linked calcium alginate gelthus returning it to a reusable form comprises at least one of: addingan acid to the cross-linked calcium alginate gel to change pH of the gelfrom basic to acidic; adding a washing fluid to the cross-linked calciumalginate gel to change the pH of the gel from acidic to neutral; oradding a salt to the cross-linked calcium alginate gel with the neutralpH to form an alginate.